1. Field of the Invention
The present invention relates to the technical field of pre-pit detection equipment for detecting pre-pits as pre-information formed on a high-density recording medium such as a DVD-R (DVD-Recordable) or DVD-RW (DVD-Rerecordable).
2. Description of the Related Art
In general, address information and reference signals for generating clock signals for use in recording and reproducing operations are pre-recorded on a recordable type optical recording medium in the form of pre-pits and pre-grooves. For example, on a DVD-R (Digital Versatile Disc-Recordable), pre-grooves as regions for recording information such as video and audio data intended to be recorded as recording pits are formed, while pre-pits (hereinafter called land pre-pits or LPP) are recorded on land portions each of which is formed between the pre-grooves.
In the operation of a pre-pit detecting device, reflected light of a light beam irradiated to a pre-groove is received in four divided regions A, B, C and D in a quadripartite photo-detector to obtain electrical signals from the divided regions A through D according to the amounts of reflected light, respectively. As a result, first and second read signals are generated on the basis of the outputs of electrical signals, and the difference therebetween is computed to generate a difference signal (hereinafter called a “radial push-pull signal”). Then the radial push-pull signal is compared with a threshold by means of an LPP-signal binarizing circuit to obtain a signal extracted as a binary signal (LPP signal). Based on the extracted LPP signal, address information and reference signals for generating clock signals for use in recording and reproducing operations are generated. Further, to improve LPP signal extraction performance, an AGC (Auto Gain Control) circuit performs AGC for the first and second read signals to make the amplitudes of both signals coincide before generation of the above-mentioned radial push-pull signal.
Upon recording record information on the DVD-R, two light beams different in intensity from each other are irradiated to the DVD-R. The light beam having high intensity (hereinafter called “recording power”) is used to form recording pits corresponding to record information on the pre-grooves so as to record the record information. On the other hand, the light beam having low intensity (hereinafter called “reproducing power”) is to read recorded information from the information tracks without forming recording pits when it is irradiated to the information tracks.
Upon recording the record information on the DVD-R, the LPPs are extracted both the period during which the light beam is irradiated onto the DVD-R with the recording power (hereinafter called the “mark period”) and the period during which the light beam is irradiated onto the DVD-R with the reproducing power (hereinafter called the “space period”).
To extract the LPPs properly during the mark period and the space period upon recording the record information on the DVD-R, there is provided a pre-pit detecting device, for example, as disclosed in Japanese Patent Laid-Open No. Hei 10-283638. In the pre-pit detecting device, an LPP-signal binarizing circuit corresponding to the beam intensity during the mark period and an LPP-signal binarizing circuit corresponding to the beam intensity during the space period are provided in parallel, so that LPP signals are extracted respectively using thresholds corresponding to respective powers of light beams. Then the logical sum of the outputs of both LPP signals is output as the final LPP signal.
The read signals input by irradiating the recording power during the mark period, however, contain lots of noise such as an electrical overshoot. Further, since the LPPs extracted during the mark and space periods vary in amplitude, proper AGC may not be performed, which causes the existence of LPP signals that will never exist in the extracted LPP signals (detection error).